Wilson cycle 1. Intracontinental rift 2. From rifting to drifting
Stages of the Wilson Cycle
Intracontinental Rifts 1. Contemporary examples (EAR, RGR, Baikal, Rhine graben) 2. Mechanical aspects. Characteristics (Regional uplift, Volcanism, Extension, Seismic activity, High heat flow, ) 3. Models of rift formation (active vs passive) 4. Failed rifts 5. Passive continental margins
Definitions Continental rift Rift system Modern rift Paleo-rift Failed Arm Aulacogen Impactogen Passive rifting New information Seismic tomography Geochemistry Deformation rates
Active vs. Passive Rifting adiabatic decompression
East African Rift Volcanic activity Plate Boundaries(Nubia, Somalia) Afar Triple junction
2 Arms of the EAR in Kenya N
The Afar Triangle N
East African Rift in Kenya
Gravity and seismic profiles
Bouguer Gravity profiles Long wavelength low -> isostatic compensation of the uplift. Short wavelength low -> low density sedimentary Short wavelength high -> magmatic intrusions
Shallow crustal structure
Seismic profile and shallow crustal structure
Lithosphere Structure-Gravity interpretation Long wavelength Bouguer gravity low compensation of topography in the mantle Short wavelengths highs: intrusions in the crust
East African rift heatflow
Earthquake depths in Kenya earthquakes are shallower under the rift axis : higher temperatures --> shallower brittle ductile transition
Tanzania craton: seismic tomography
Afar triple junction: EAR-Red sea-gulf of Aden
Continental flood basalts
Evolution of the Afar triple junction Remember paleomagnetism shows rotation of Danakil block relative to Africa
Western US: Basin and Range, Colorado Plateau, Rio Grande Rift
B&R Colorado Plateau RGR N
Rio Grande Rift
Seismicity of the western US
B&R, RGR magmatism (a) Mesozoic (b) Miocene (c) Pliocene (d) Quaternary Note: little activity in CP NW (Yellowston e hotspot)
Western US Normal Faults
B & R extension (GPS) (a) GPS velocity across the Basin and Range, western United States with respect to North America (blue vectors) with 95% confidence ellipses superimposed on topography (Lambert conic projection). Confidence ellipses include uncertainty in the North America reference frame. (b) Expanded view of faults around the Central Nevada Seismic Zone. Faulting is shown with colored lines: cyan (historic), magenta (Holocene), and purple (Late Quaternary).
North America Heat flow map
RGR seismic travel time residuals Map of the southwestern United States showing the location of the La Ristra array seismic stations. Dashed lines show boundaries of two Proterozoic provinces from Karlstrom and Humphreys [1998].
RGR travel time residuals The P and S models obtained through inversion of travel time residuals. Slower regions are shown in red colors and fast regions in blue.
RGR mantle seismic tomography and receiver function showing velocity discontinuities
RGR deep structure
Chronometry of EAR and RGR Rio Grande Rift 40-30 Ma calc-alkaline volcanism Subduction of Farallon plate. 32 Ma rhyolitic volcanism 30-20 Ma Bimodal volcanism (rhyolites-basalts) ~20 Ma Ridge off California is subducted. San-Andreas transform fault begins. East African Rift 50-30 Ma basalts 45 Ma Flood basalts 21-14 Ma Flood basalts 16 Ma basalts 15-11 Ma Regional Uplift (500m) 12 Ma rhyolites 10 Ma Western shoulder uplift 8 Ma Faulting 7-5 Ma Ma Alkaline basalts 6-2 Ma Flood lavas 8-0.5 Ma Flood basalts (2 episodes) 4 Ma Main uplift (1,500m) 3 Ma Dykes 2 Ma Normal faulting (graben) 1 Ma Plateau uplift 1 Ma present Volcanism-Faulting
Baikal rift (<20 Ma)
Baikal seismicity
Heat flow
Baikal mantle structure Two-dimensional teleseismic tomography image: (a) averaged residual times (P and PKP waves), (b) target area with blocks, and (c) velocity cross-section. The low-velocity area in the mantle beneath stations 11, 12, 24, 21, 22, and 23 corresponds to the asthenospheric upwarp beneath the Baikal rift zone. The low velocities in the southeastern part of the cross-section (from stations 84 to 89) can correspond to a plume head beneath the Hentey dome
Rhine Graben European Cenozoic (~60 Ma) rift system and tectonic setting of the Upper Rhine Graben in the northern Alpine foreland. Cross hatched, Variscan basement outcrops on the European platform; shaded, Cenozoic rift deposits; dotted, Alpine Molasse; dark, Cenozoic volcanics. BG, Bresse Graben; BTZ, Burgundy Transform Zone; EG, Eger Graben; HG, Hessian Grabens; LG, Limagne Graben; LRG, Lower Rhine Graben (Roer Valley Graben); RG, Rhône Graben; URG, Upper Rhine Graben; VB, Vogelsberg volcano. Insert rectangle shows study area.
Moho depth
Rhine graben heatflow
Gulf of Corynth is just another example
General remarks Rhine graben and Baikal rift are near collision zone. Stresses are induced by collision and by crustal and lithospheric thickening BR and RGR development coincide with the termination of the Farallon plate subduction Only EAR is a pure rift?
General characteristics of all rifts Broad uplift Crustal thinning Bouguer anomaly (long wavelength low, +short wavelengths) Seismic tomography (low velocity anomaly in upper mantle localized) High heat flow localized (with short wavelengths variations) Seismicity + volcanic activity
Rifting Continental extension, breakup Sedimentary Basins Continental Margins
Stages of the Wilson Cycle 1. Intracontinental rift: continental extension, graben formation, volcanic activity 2. From rifting to drifting: oceanic crust formation, central rift formation 3. Evolution of oceanic basin: Creation of new ocean floor at ridge 4. Initiation of subduction: Ocean-ocean subduction, Formation of insular arcs, Ocean-continent subduction 5. Closure of the ocean basin Continental Collision: Building of mountain belts
Example : East African Rift Afar East African Rift, Afar Triple Junction (RRR type) Kenya/Tanzanie
Mantle seismic structure of Rifts : example : EAR P-wave velocity S-wave velocity Continental Rift Afar Transitional Zone Bastow et al., 2005, Geophys. J. Int.
Mantle seismic structure of Rifts : example : Baikal Zhao et al., 2006, Earth Planet. Sci. Lett. 243
Passive or Active Mechanism? Remontee adiabatique Passive : tectonic forces drive extension; then asthenospheric upwelling Active : asthenospheric upwelling occurs first, this drives extension
Passive vs Active Rifting Passive Plate stresses -> extension Crustal and lithospheric thinning - > uplift? Active Hot spot activity -> volcanism Asthenosphere penetrates lithosphere -> Uplift Uplift -> Extension Adiabatic decompression -> volcanism and possible
Mechanisms of Rifting Active Thermal anomaly in mantle (hot spot, plume) Magmatism Uplift of the region Extension Rift Formation Passive Extension due to stresses in the plates Thinning of crust and lithosphere
Proposed rifting mechanisms East African: probably active mechanism Rio Grande: délamination mechanism? Baikal/Rhine: associated with continental collision
Active rift model
Delamination Delamination involves the removal and sinking of lithospheric mantle and replacement by asthenosphere It could have happened beneath B&R
Graben valley formation In rifts, grabens are surrounded by symmetric normal faults Why graben depressed relative to horst? Why central graben often symmetric with uplifted shoulders?
How much extension in the rift zone? Basin & Range In rifts, displacement on normal faults (for 60 deg, vertical / horizontal ~ 1.7 => a few km at most (10-20 %). In rifts, crustal thickness (30km vs 40km) => at most 25%. In Basin and Range, extension is often on listric or low
Extension much larger in B & R Estimates are 100% extension for B&R. Note this implies crust was 60km before extension. (Post orogenic collapse?) Metamorphic «core complex» formation faulting and isotstatic uplift lower crustal outcrops. Typical of Basin & Range.
Extension Models Pure Shear: décollement produced at interface between brittle and ductile zones. Above the décollement, extension is controlled by normal listric faults normales Below, extension causes flow of lower crust
Simple Shear décollement persists throughout the crust, then is transformed to shear ductile zone Rift is generally asymmetric, possibly due to disequilibrium between surface processes and deeper structure
Temperature and lithosphere extension
Rifting to Drifting : When Extension continues
Plumes and rift Hotpots/plumes often initiate extension -> continental fragmentation (ex. separation of modern continents from Gondwana supercontinent)
Opening of ocean: link with hot spots and rifts.
Effects of temperature and extension factor on the formation of oceanic crust along continental margins and ocean basins. Oceanic crust represents infinite stretching. (White & McKenzie, 1995, J. Geophys. Res. 100)
Melting at MOR
Mantle T determines % melting Oceanic crustal thickness is uniform because it is fixed by mantle temperature. Exception at hotspots on MOR. Very thick crust in Iceland. Mantle temperature was higher during the Archean.
Thick crust for the Iceland hot spot
And if Rifting fails? Atlantic failed arms and triple junctions Text Failed arm = Aulacogen Hypothesis: continental rupture initiates a series of triple junctions RRR (trois zones de rift). Oceanic basin is created by 2 boundaries, the other fails. Many examples in the Atlantic margins, and also in the continental interior, from the Precambrian to present day.
Failed rifts in North America
Filtered Gravity map (40km -> 200km) Failed Rift : Keweenawan v
Keweenawan rift system
-Keweenanwan is the deepest rift on Earth - Seismics show very thick volcanics and sediments
Aulacogens in eastern Canada Saint-Laurence Valley, Great Lakes, Ottawa-Bonnechere Graben, and the continuation of the Outaouais Valley of Lake Témiscamingue are examples of failed rift zones. These zones are associated with the rupture of the Laurentia supercontinent.
Adams & Basham, 1991; «The seismicity and seismotectonics of eastern Canada») shows emplacements of primary aulacogens in eastern Canada. Note: region labelled «Extinct ridge and transforms» probably continues in Baffin Bay.